1. Field of the Invention:
The present invention relates to improved cationic compositions and to the use of these compositions in cationic electrodeposition. More particularly, the present invention relates to cationic compositions containing a material which contains free formaldehyde and/or is capable of generating free formaldehyde.
2. Brief Summary of the Prior Art:
Cationic compositions for use in electrodeposition are well known in the prior art. Such compositions are used extensively in the automobile industry for applying primer coats to automobile and truck bodies. Typical of the prior art describing such compositions and their use in electrodeposition is U.S. Pat. Nos. 3,799,854; 3,947,338; 3,947,339 and 3,984,299.
Cationic electrodepositable compositions containing a material prepared from formaldehyde and containing methylol or substituted methylol groups such as aminoplast resins and phenoplast resins are also well known in the art. Such materials can be used for curing or they can be present in the compositions as an additive for the purpose of plasticizing, improving adhesion or imparting other desirable effects to the coating. Prior art which describes the use of such materials in cationic electrodepositable compositions are U.S. Pat. Nos. 3,663,389 and 3,937,679.
Surprisingly, it has been found that cationic compositions, particularly those containing primary and/or secondary amino groups in combination with materials prepared from formaldehyde-reactive material and containing methylol or substituted methylol groups such as aminoplasts, often exhibit a lowering of the pH with time. The lowering of pH has been found to be due to the presence of free formaldehyde in the composition. Although formaldehyde is not itself acidic, it may be converted to formic acid by any of several mechanisms such as oxidation or disproportionation. Also, the formation of aminals from the reaction of formaldehyde with primary or secondary amines can result in a lowered pH. This makes the compositions more corrosive with the ferrous metal parts such as the electrodeposition tanks and attending piping and pumps in contact with such compositions. The formaldehyde can be present with the aminoplast but even with low free formaldehyde-containing materials, can be generated by hydrolysis of aminoplast. It has been found that lowering of the pH can be abated and the pH stabilized by adding to the cationic compositions a formaldehyde scavenger.